The present invention relates to a beam splitter assembly and to an interferometer using the beam splitter assembly.
Interferometers generally include one or a plurality of beam splitter assemblies used to physically split a beam of rays into a measuring path and a reference path, often referred to as the measuring arm and the reference arm. This assembly is generally used to combine the measuring path and the reference path in the opposite direction of the beam. The optical axes of the measuring arm and reference arm are oriented differently relative to one another, depending on the type of interferometer. Thus, the measuring arm and the reference arm may be essentially perpendicular to one another or parallel to one another.
In the former case, beam-splitting prisms onto which a triple prism that creates the reference arm is cemented are generally used. This is shown, for example, in U.S. Pat. No. 4,802,765.
In the latter case, where the measuring path and the reference path are parallel to one another, a Kxc3x6sters prism is usually used as a suitable beam splitter. A prism of this kind used as a classic beam splitter element is described in the textbook Bauelemente der Optik [Design Elements in Optics], Naumann/Schrxc3x6der, Carl Hanser Verlag, 6th Edition, page 184. The principal disadvantage of a beam splitter of this kind is that it is relatively expensive to manufacture.
Any manufacturing-related pyramidal flaws in the Kxc3x6sters prism cause the path to diverge from the ideal path to a greater or lesser degree. The parallelism of the two outgoing beams is sensitive to whether the angle of incidence of the incoming beam relative to the beam splitter assembly is optimally correct. Even a slight angle relative to the optimal axis of incidence causes the two outgoing beams to converge or diverge significantly. Similarly, if the incoming beam undergoes a parallel shift relative to the optimal axis of incidence, this has a significant effect on the distance between the two outgoing beams. In particular, problems occur if the path length between the beam splitter assembly and the downstream measuring reflector and reference reflector in the measuring arm and reference arm is large. Changes in the path length along the direction of propagation of the beam exiting the beam splitter assembly are disadvantageous, as optical components arranged downstream in the beam path must be oriented precisely to these beams. As a result, adjustments involving considerable expenditure must be made to ensure that the incidence of the incoming beam is optimally correct.
U.S. Pat. No. 5,675,412 describes a beam splitter assembly that avoids these problems. The two components, one of which has a beam-splitting and the other a beam-deflecting optical effect in the assembly, are arranged separately from one another, and must therefore be mounted with great precision relative to one another, so that the two outgoing beams propagate parallel to one another.
U.S. Pat. No. 5,808,739 describes another type of beam splitter assembly for an interferometer that delivers two beams that are parallel to one another. Herein, an incident beam impinges on a first arrangement that includes two planar plates arranged parallel to one another and fixed in position using spacers, so that they face one another. On the rear side of the first planar plate, a beam splitter layer is arranged which splits the input beam into a transmitted beam component ART and a reflected beam component ARR. Reflected beam component ARR exits the first plate after passing through it again and propagates to a second arrangement that includes two spaced planar plates. After being reflected a plurality of times in the second arrangement, reflected beam component ARR exits the beam splitter arrangement in the direction of the reflector. Beam component ART, which has been transmitted through the beam splitter layer of the first arrangement, is reflected off a second reflective layer of the second planar plate and deflected back in the direction of the first planar plate. After passing through the first planar plate again, beam component ART exits the first planar plate arrangement parallel to beam component ARR.
In order to achieve the advantages associated with this type of beam splitter as proposed, the two planar plates must be oriented parallel to one another within the first arrangement with great precision. To accomplish this, adjustments involving considerable expenditure are required during assembly, and the spacers arranged between the planar plates are also expensive to manufacture.
The present invention is thus a beam splitter assembly, in particular for an interferometer, that splits an incoming beam into at least two outgoing parallel beams. The device is as insensitive as possible to non-optimally oriented incoming beams, and is inexpensive to manufacture compared to current devices. The invention is also an interferometer that uses the beam splitter assembly and prevents the aforementioned problems of the current devices from occurring.
In one embodiment, the invention is a beam splitter assembly for an interferometer adapted to split an incoming beam into at least one first outgoing beam and at least one second outgoing beam parallel to the at least one first beam. The beam splitter includes at least one beam splitter element, each of the at least one beam splitter elements further comprising a transparent plate having two parallel boundary surfaces, a first of the two boundary surfaces having a portion functioning as an at least partially reflective reflector surface, and a second of the two boundary surfaces having a portion functioning as a beam splitter surface. The beam splitter also includes at least one compensating element disposed relative to the at least one beam splitter element so that the at least one first and second outgoing beams propagate parallel to one another, the at least one compensating element having dimensions adapted to provide a substantially identical optical path length in the at least one beam splitter element as in the at least one compensating element to the at least one first and second outgoing beams.
In another embodiment, the invention is an interferometer having at least one measuring arm and a reference arm, comprising a light source, at least one stationary reference reflector in the reference arm, at least one measuring reflector movable in a measuring direction in the measuring arm, a detector unit for detecting a position-dependent interference signal, an evaluation unit for processing the interference signal detected by the detector unit, and at least one beam splitter assembly for splitting an incoming beam emitted by the light source into at least one first outgoing beam and at least one second outgoing beam parallel thereto, said at least one beam splitter assembly further comprising. The beam splitter assembly also includes at least one beam splitter element comprising a transparent plate having two parallel boundary surfaces, one of these boundary surfaces having a portion functioning as a beam splitter surface, and a further boundary surface parallel to the aforementioned boundary surface having a portion functioning as an at least partially reflective reflector surface, and at least one compensating element arranged with respect to the at least one beam splitter element so that at least two outgoing beams propagate parallel to one another, and so that the at least two outgoing beams travel along optical paths having substantially the same length in the interferometer before reaching the detector unit.
The beam splitter assembly according to the present invention includes two separate elements, namely the beam splitter element and a compensating element. Both elements of the beam splitter assembly can be plane-parallel plates, so that no extra manufacturing expenditure or adjustment is required to ensure that the parallelism requirements associated with the respective boundary surfaces are met.
Furthermore, the beam splitter assembly according to the present invention ensures that if the incoming beam is not optimally oriented, the at least two outgoing beams propagate substantially parallel to one another and the distance between the two outgoing beams is kept substantially constant.
The proposed beam splitter assembly and the associated adjustment tolerance allows a certain amount of flexibility when designing an interferometer according to the present invention. In particular, the interferometer measuring head may be modular in design and non-critical in terms of adjustment, and may include an optics module and an electronics module. The optics module can contain, among other things, the beam splitter assembly according to the present invention. The electronics module can contain, among other things, the detector elements for detecting the interference signals.
The interferometer according to the present invention may be designed differently for different applications, for example for combined detection of linear and rotational movements.